Ages and evolution of early–type cluster galaxies at z ∼ 1 from the ACS intermediate redshift cluster survey

HST permits the study of high–z cluster galaxies in unprecedented detail and offers a unique opportunity to examine the processes driving galaxy formation. As part of our Advanced Camera for Surveys (ACS) intermediate redshift cluster survey, we have observed 8 galaxy clusters with redshift betwen 0.8 and 1.3. At these redshifts, galaxy clusters are still assembling and galaxies are evolving towards the populations that we observe today. Recent results from the ACS cluster survey have shown that galaxy clusters at redshift around unity show differences but also many similarities with local clusters, in terms of galaxy populations and their distribution. One of the most striking similarities is that the tight relation between galaxy colors and luminosities that applies to local early-type galaxies (the colormagnitude relation; CMR) is already in place at redshifts as high as z∼ 1.3. The very precise measurement of the scatter around the CMRs of our cluster sample permits us to constrain the formation ages of the redder early-type populations to be at z > 2.5. We will present our most recent results and discuss them as a function of cluster properties at z ∼ 1.